The present invention relates to a method according to the preamble of claim 1.
In the prior art, there are known several different methods for making joints between stainless steel and copper. When joining copper and stainless steel together, several factors must be taken into account, among them the different strengths of the materials in high temperatures, as well as their differences in thermal conductivity, thermal expansion and melting temperatures. In joining methods, such as soldering or welding, problems are caused by so-called soldering brittleness, which means the penetration of copper in a diffusion joint to the grain limits of steel, which makes the joint remarkably brittle. The U.S. Pat. No. 4,194,672 discloses a method for joining copper and stainless steel by diffusion welding. In said method, a thin layer of nickel, nickel alloys, chromium, Ni+Cr or Cr+Ni, is used in between the junction surfaces of the pieces to be joined in order to prevent copper from being diffused to the grain limits of stainless steel and to improve adhesion. The joint is diffusion welded at a temperature of 850xc2x0-950xc2x0 C. The method requires a relatively high temperature and a long period of thermal treatment in order to create the joint. Moreover, the strength of the joint achieved by means of this method is limited. The U.S. Pat. No. 5,664,723 introduces a soldering technique where a piece of copper is joined to metal or to a ceramic material. The joint of copper and stainless steel disclosed in said publication is made by using an extremely thin nickel layer as a diffusion barrier on the steel surface, and in addition by using a tin layer as a soldering agent on the copper surface. The pieces are brought together and heated in two steps, first up to 400xc2x0 C. for the duration of one hour, and after that additionally up to 800xc2x0 C. for the duration of 20 minutes. Thus the applied method requires a relatively long heating period in two steps.
The object of the present invention is to realize a method for making a joint whereby the drawbacks known in the prior art solutions can be avoided. Another object of the invention is to realize a joint that maintains good electroconductive capacities even in corrosive conditions of extensive duration. Yet another object of the invention is to realize a joint that has a good resistance in high temperatures.
The invention is characterized by what is specified in the appended claims.
The method according to the invention is mainly characterized in that a first intermediate layer is brought onto the junction surface of the steel object or against said surface, mainly in order to prevent the loss of nickel from the steel object, and at least a second intermediate layer onto the junction surface of the copper object or against said surface in order to activate the creation of a diffusion solution. According to a preferred embodiment, the first intermediate layer is chosen from the following group: nickel (Ni), chromium (CR), a mixture or alloy of nickel and chromium (Ni+Cr), a mixture of chromium and nickel (Cr+Ni). According to a preferred embodiment, the second intermediate layer mainly consists of tin (Sn).
According to a preferred embodiment of the invention, there is additionally arranged at least a third intermediate layer in between the first intermediate layer and the copper object. According to a preferred embodiment of the method, there is brought at least a second intermediate layer and at least a third intermediate layer, and the melting temperature of the second intermediate layer is lower than that of the third intermediate layer. The third intermediate layer may consist mainly of silver (Ag), or it can be an alloy or mixture of silver (Ag) and copper (Cu). According to a preferred embodiment of the method, the junction area is heated in one step.
The method according to the invention has several important advantages. By applying a nickel layer on the steel surface when making the joint, it is possible to prevent a nickel loss that could take place from the austenitic stainless steel towards copper and could embrittle the steel. The creation of the joint is activated by means of a soldering agent layer at the junction surface between the copper surface and the nickel-plated steel sheet. By means of the activator, lower joining temperatures can be used, so that the thermal stresses created in the junction area are smaller. By using the intermediate layers according to the invention, a one-step heating of the junction can be applied in the creation of the joint. By means of the arrangement according to the invention, there are achieved joints that have a better tensile strength than the conventional diffusion joint.
In this application, the term copper refers to, apart from objects made of copper, also to alloy materials with a copper content that essentially includes at least 50% copper. The term stainless steel in this application refers mainly to austenitic alloy steels, such as stainless and acid-proof steels.